A　sound-absorbing　metasurface　with　a　multi-slit　synergetic　resonant　effect　was　studied　that　can　achieve　broadband　sound　absorption　in　a　range　betwen　500　Hz　and　1150　Hz　by　using　a　nested　doll　supercell　with　a　thickness　of　only　44.1　mm　(the　thickness　being　one　sixteenth　of　the　wavelength).　The　basic　unit　of　the　improved　micro-slit　absorber　(IMSA),　made　by　designing　the　slit　of　the　micro-slit　absorber　as　a　type　of　step　and　locating　it　at　the　edge　of　the　surface　panel,　is　proposed　for　sound　absorption　at　lower　frequencies.　The　improved　step　slit　can　not　only　change　the　structures　impedance　to　match　the　medium　of　air　at　a　lower　frequency,　but　can　also　guide　sound　waves　into　the　slit　and　increase　the　air　velocity　step　by　step,　which　results　in　a　strong　friction　loss　in　the　lower　micro-slit.　On　this　basis,　a　nested　doll　supercell　with　eight　integrated　parallel　IMSAs　was　designed　and　tested,　and　was　able　to　achieve　sound　absorption　in　a　range　between　500　Hz　and　1150　Hz.　In　addition,　the　bandwidth　was　further　broadened　(to　between　330　Hz　and　1150　Hz)　by　adding　four　additional　IMSAs　to　the　previous　supercell,　while　the　surface　area　of　the　panel　only　increased　by　about　20%.　This　work　highlighted　the　potentially　extensive　application　of　broadband　noise　reduction　due　to　its　depth,　i.e.　subwavelength　thickness,　and　the　relatively　small　surface　area　of　the　panel.　©　2020　IOP　Publishing　Ltd.